Magneto-optic light rotator



350-376 0 SR Search R001 0R 2 4 2 i396 I wzw/ K June 948- F. w. BUBBETAL 2,442,395

lAGNETO-OPTIC LIGHT ROTATOR Filed m 5, 1943 I 2 Sheets-Sheet 1 7 7 E 1Bl! I 18 1f ooooobo f BEAM A 30 10 f 12 14/ 24 V ooooooo [22 2/922 areFRANK w. 5055 A. 1.. 01/1/44 D'ADR/A/V fltforney Search R00 June 1,1948. F. w. BUBB EI'AL 2, v "AGNETO-OPTIC LIGHT RbTATQR Filed lay 5,1943 2 Sheets-Sheet 2 [In/en fora FRANK M45055 ,4. L. Dav/1L DADR/A/V Htforzzey Patented June 1, 1948 Search R UNITED STATES PATENT OFFICEMAGNETO-OPTIC LIGHT ROTATOR 1943, Serial No. 485,962

Application May 5,

4 Claims. I 1

This invention, originally believed to be an integral part of theinvention described and claimed in copending application Serial No.401,610, which issued February 8, 1944 as U. S. Patent No. 2,341,422,was found to be a separate invention. The combined invention was a jointinvention, as is the invention in the present case; but the inventionclaimed in U. S. Patent No. 2,341,422 is the sole invention of Frank W.Bubb.

This invention relates to improvements in photoelastic instruments. Moreparticularly, the invention relates to photoelastic instruments that areused to determine accurately the stresses in test models.

It is an object of the invention to provide an improved photoelasticinstrument that may be used to determine accurately the stresses in testmodels.

Various kinds of photoelastic instruments have been known and used forsome time. These instruments include polariscopes of various kinds andvarious types of compensators. Scientists have used these instruments toindicate the presence of stresses in test models. At first, theinstruments could only be used to determine the existence of stressesqualitatively, but now the instruments can be used to determine stressesquantitatively. The present invention provides a novel photoelasticinstrument that quantitatively determines the existence of stresses in atest model and does so quite accurately. The invention can do thisbecause it provides a novel form of photoelastic instrument. Thisinstrument uses a device to rotate the light. By providing aninterferometer that has a light rotating device, it is possibl to obtainan exceedingly accurate photoelastic instrument. It is, therefore, anobject of the present invention to provide an interferometer having alight rotating action.

Various kinds of light rotators have been made and used, but none ofthem are accurate enough for use in an interferometer. To be useful inan interferometer, a light rotator must give rotation of light that isconstant within one per cent over the whole field of view of therotator. The invention provides such a rotator and it is, therefore anobject of the present invention to provide a light rotator that givessubstantially constant rotation of light over the entire field of viewof the rotator.

Light rotating devices of two general types have been made. These areoptical light rotators and magnetic light rotators. Although several ofthese have been made, no magnetic light rotator has been suflicientlyaccurate to be used suc- 2 cessfull in an interferometer. Magneticrotators are not new, but the particular one provided by the inventionis novel. Magnetic light rotating devices consist of a coil that inducesa magnetic field in a core of transparent material. The core may be oneof a number of different materials that have a magnetic opticalrotational effect. The difierent materials have different Verdetsconstants. An examination of a table of the various transparentmaterials which have a magneto-optical rotational eifect indicates thatthe substances having a low Verdets constant are usually stable and thesubstances having a high Verdets constant are usually unstable or areslightly opaque. To be usable in a light rotator, a substance must havea relatively high Verdets constant, and it should be stable. Manymagnetic light rotators use carbon disulfide as the core because it hasa fairly high Verdets constant and is fairly stable. To secure a 45rotation of light with carbon disulfide or any of the usual transparentcores, a coil that is several feet long or one carrying an excessivecurrent would have to be used. Such a coil would have to give a magneticfield that was uniform along its radius and the coil would have to bekept cool. It would be exceedingly diilicult if not absolutelyimpossible, to make such a coil. It is possible, however, to make apractical magnetic light rotator by treating a relatively unstablesubstance, that has a high rotational effect, to make it stable. Such asubstance is mercuric potassium iodide which has a much higher Verdetsconstant than carbon disulfide. This substance has heretofore not beenused because it decomposes when subjected to light. By using a treatedsolution of this substance ior the core of a magnetic light rotator, itis possible to make a practical magnetic light rotator. It is,therefore, an object of the present invention to provide a treatedsolution of mercuric potassium iodide that is usable with a magneticlight rotator.

The magnetic field of a magnetic light rotator will not have a constantstrength radially. The center of the core will be subject to a magneticfield that has a strength difierent from the magnetic field at the edgeof the core. The difference in the magnetic fields at the difierentparts of the core, will affect the rotation of th light. This is theresult of the magneto-optical law that the strength of the magneticfield and the length of the path through the magnetic field determinethe amount of rotation given. We have invented a method that enables usto provide a number of co-axial paths through the core that havedifferent effective light rotational lengths. The lengths of theco-axial paths are made so the product of the magnetic intensity for aparticular path times the length of the particular path is equal to theproduct of the magnetic intensity of any other path times the effectivelength of the other path. Constant rotation of light over the entirefield of the rotator can be attained in this way. It is, therefore, anobject of the present invention to provide a number of co-axial pathsthrough the core having different effective light rotational lengths.

Other objects and advantages will appear from the drawings andaccompanying description.

In the drawing and accompanying description, a preferred embodiment ofthe invention is shown and described, but it is to be understood thatthe drawing and accompanying description do not limit the invention andthe invention-will be defined by the appended claims.

In the drawing:

Fig. 1 is a schematic diagram of the photoelastic instrument when it isused as an interferometer.

Fig. 2 is a schematic diagram of the photoelastic instrument when it isused as a polariscope, and

Fig. 3 is an enlarged cross-sectional view of the magnetic light rotatorprovided by the invention.

Referring to the drawing in detail, a source of light is denoted by thenumeral I6. Positioned in proximity to the source of light I is a lens[2. A polarizer i4 is located near the lens l2 and is oppositelydisposed with relation to the source of light ID. A quarter-wave plateI6 is removably positioned in proximity to the polarizer l4. Ahalf-mirror I8 is positioned beyond the quarterwave plate I6. A planereflecting mirror is denoted by the numeral 20. A second quarter-waveplate is denoted by the numeral 22 and it is located in proximity to ananalyzer 24 and a screen 26. A test model and a strain frame in which itis stressed, are denoted by the numeral 28. A magnetic light rotator isgenerally denoted by the numeral 36. This magnetic light rotator 36generally consists of a tube of transparent liquid and a solenoid. Thetube for the transparent liquid may be of any suitable design, but Iprefer to use an enameled container 32 with glass ends 34 and 36. Wheredesired, the glass end 34 of the container may be coated with aquarter-wave film of barium stearate or otherwise treated to increaseits transparency. A coating of barium stearate increases the lighttransmitting ability of the glass end 34 of the container 32, andeliminates ghosts. The connection between the glass ends 34 and 36 andthe enameled container 32 is tight enough to permit the container 32 tohold liquids. This liquid can be inserted into the container 32 throughpipes 33 and 40. The glass end 36 of the container 32 is silvered tomake the end 36 a mirror. The container 32 is mounted in a tube 42. Thistube 42 is longer than the container 32 and is supported by end rings44. The end rings 44 are supported by adjusting screws, not shown, thatpermit the accurate'adiustment of the photoelastic instrument. The tube421s covered with heat insulating material 46 and this material isenclosed in an equalizing tube 48. A water jacket 56 encircles theequalizing tube 48, and is spaced therefrom a short distance to form anair space between the tube 48 and the Jacket 50. Pipes 52 are connectedto the water jacket 56 and permit water to be circulated through thejacket. A number of spirally wound solenoid coils 54 are supported onthe water Jacket 63. "These 4 coils 54 are separated from each other byfins I6 that contact the water jacket 56.

In the operation of the photoelastic instrument, two distinct proceduresare followed. The first is the use of the instrument as aninterferometer to obtain the isopachic lines and the second is the useof the instrument as a polariscope to obtain the isochromatic andisoclinic lines. Having obtained these lines, it is easy to calculatethe stresses in the test model. When the instrument is used as aninterferometer, the quarterwave plates I8 and 22 are removed, and themagnetic light rotator is actuated. The operation of the instrument asan interferometer is as follows:

The light source 10 emits monochromatic un- 1 polarized light thatpasses through the lens l2 and is thereby rendered parallel. Theparallel beam from l2 passes through a polarizer l4 and is thereby planepolarized. The plane polarized beam falls upon a half mirror I8 whichhas optically parallel faces. The half-mirror i8 divides the light beaminto two beams A and B of equal intensity. One of these beams A, passesthrough a transparent model 28 that is held in a strain frame. Thisstrain frame can be arranged to apply a given set of external forces tothe model 28. These external forces induce a stress distribution in themodel which can be quantitatively determined by the photoelasticinstrument provided by the invention. The light vector E1 of the beam Apassing through a point of the test model 28 where the principalstresses are P and Q, is resolved by stress-induced double refractioninto two plane polarized beams. One of these plane polarized beams whichwe denote by the letter 9. has its light vector parallel to theprincipal stress P. The second of the two plane polarized beams which weshall denote by the letter q, has its light vector parallel to theprincipal stress Q. Since P and Q are at right angles, the two planepolarized beams 12 and q are at right angles to each other. The beamdenoted by the letter 1) and the beam denoted by the letter q, proceedthrough the model 28 and suffer alterations in their optical paths dueto two causes.- First, the refractive index a of the model is altered bythe stresses P and Q. This effect is given by the stress-optic lawHP-M=C1P+ E uation 1 tQt=c.Q+c.P q

where 01 and C2 are constants. Second, the

principal stresses P and Q change the thickness of the test model 28 inaccordance with the elastic law Q)? Equation 2 where 6t is the change inthickness t, v is Poissons ratio. and E is Youngs modulus. Therespective changes in optical paths as compared with path through theunstressed model are:

From Equations 1, 2, and 3, we have ssP= aP+ aoflt+ 011 c.Q P+Qwe(awaiti-o-acwwxmofi:

- Equations 4 583K) KOl 2,442,.aee

This last equation expresses the changes inthe optical paths for thebeams p and q in terms of the stresses P and Q. After traversing themodel 28, the beams p and q pass into the light rotator 30. During theirinitial passage through the rotator 30, the beams are rotated 45 in thedirection in which the positive current flows in the coil. At the end ofthe rotator is a silvered plate 38 that reflects the beams p and q. Thereflected beams will pass back through the rotator and will be rotatedan additional 45. Thus beams p and q return to the model 28 with a 90rotation of their light vectors. In passing through the model 28 afterrotation, the roles of the beams p and q are interchanged. The beam 2)suffers the retardation 68a in its optical path and the beam q suffers aretardation 68p. Upon emerging from the model after rotation, both beamsp and q have the same optical retardation.

( 1+ C2) QP t Equation 5 In particular the beams p and q have norelative optic rotation. Hence the beams 19 and q recombine into a lightvector E} at right angles to the original direction of th light vectorEr. In this manner all the light that emerges after its second passagethrough the model 28 is rotated at 90 to the original direction,

- regardless of the orientation of P and Q. A calculation of the twoterms in the preceding equation, Equation 5, from known experimentaldata for Bakelite, shows that the second term is of the order of 0.005times the first. Neglecting the small term, we have a high precisionBakelite was used as being representative of the synthetic resinscommonly used in making the test models in photoelastic work. The aboveequation, Equation 6, gives, relative to the optical path through theunstressed model, just that change in path which is produced by thestresses P and Q. The reflected beam A that emerges from the test model28 after being rotated, strikes the half-mirror 18 where one-half of thebeam passes through and is lost and the other half is reflected to ascreen or photographic plate 26. The other half of the light from thelight source that strikes half-mirror I8 is reflected by thehalf-mirror. This light is designated as beam B and it contacts a mirror20 and is reflected back to the half-mirror l8 where half of it is lostby reflection and the other half passes through. The half that passesthrough half-mirror l8 enters analyzer 24 which is interposed betweenthe halfmirror l8 and the screen 26. Because the light vector and E2 ofcorresponding parts of the beams A and B are at right angles to eachother, they will not ordinarily interfere. To produce interference,however, we interpose the analyzer 24 between half-mirror l8 and screen26 which has its axis at 45 to the light vectors of the beams A and B.The analyzer 24 permits only those components to pass that are parallelto the axis of the analyzer. Those parts of the beams A and B that aredirected toward the screen 26. have parallel light vectors and are inproper condition to interfere. Any difference in optical paths betweenbeams A and B when beam A passes through the unstressed-model can beignored since interference upon the screen 28 due to such diflerence inoptical paths, would produce a uniform effect all over the screen if theinterferometer is adjusted properly. It then appears that 65 of Equation6 is the only part of the path difference between corresponding parts ofthe beams A and B that needs to be taken into account. This pathdifference as is due entirely to the effect of the stress sum P plus Q.If the interferometer isinitially adjusted with the model 28 unstressedto a uniform extinction over the whole screen 28, it follows that,complete extinction occurs for all points of the stress model for whichis an integral multiple of the wave length A of the light. For points ofthe model 28 where a is an odd multiple of M2, the corresponding partsof beams A and B reinforce each other on screen 28. Thus the screen orphotographic plate 28 will show a set of alternating bright and darkinterference fringes. Lines may. of course, be interpolated between anytwo dark fringes corresponding to any values of fringe wanted.

Hence if we set 6S= mg Equation 7 so whatever the value of m, we have,upon combining with Equation 6, the important result, P+Q=Im, where I isa constant which may appropriately be called the isopachic fringeconstant. The isopachic constant I is, in fact, given by although thisis of little importance since I can be measured for a given material, agiven thickness t, and a given wave length A by an obvious tension test.The important point here is that the instrument provided by theinvention reduces the problem of flnding the stress sum P+Q, at anypoint of the model to the mere counting of fringe order. This is easilydone by watching the formation of the fringe path as the stresses areapplied to the model. For a Bakelite model .25 inch thick, the isopachicfringe constant I turns out to be, for the double passage of lightthrough the model, about 85 pounds per square inch per fringe.

The second operation of the instrument is its use as a polariscope togive the isochromatic and isoclinic lines. 0nly beam A is needed for thepolariscope and beam B that is reflected from the half-mirror l8 may beabsorbed on a black screen l9 interposed between mirror 20 andhalf-mirror l8. As in the case of the interferometer, the light from thelight source l0 passes through lens l2 and is rendered parallel. Thisparallel beam passes through polarizer l4 and is thereby made planepolarized. The beam then passes through a quarter-wave plate l8, theaxis of which is set at to the plane of polarization. The quarterwaveplate l8 circularly polarizes the light and this light passes throughthe half-mirror l8 to the model 28. The passage of the beam A throughthe model 28 produces the two beams p and q whose path differences aregiven by Equation 4. When the instrument is used as a polariscope, therotator is not actuated and the light entering the rotator is reflectedback without rotation. 76 When the light passes back through the modelterfere.

23, the relative ath diiferencu are doubled instead of being equalizedas in the case of the interferometer. The beams p and q proceed to thehalf-mirror II where half of them is lost and the other half isreflected downward. This reflected light passes through a secondquarterwave plate 22 that converts each beam into a plane polarizedbeam. The plane polarized beam passes through the analyzer 24 that iscrossed with polarizer ll. Since the analyzer 2i transmits only thosecomponents of beams p and q which have light vectors parallel to theaxis of the analyzer 24, the portions of the beams p and q which strikescreen 26 are in condition to in- The path difference between these twointerfering beams p and q is twice the difference 6SP-6S between thevalues given in Equation 4, namely,

In the last, the second term on the right is of Order .002 times thefirst, and hence negligible. To a high precision than Reasoning asbefore, we see that alternate bright and dark fringes will be cast upona screen. When aS=n where n is a whole number, we have a dark fringe.When n is of the form 2k+1, where k is a whole number, we have a brightfringe. For all values of n then we have upon combining aS=m. withEquation 9,

P-Q=Kn where '1 i) is a constant called the isochromatic fringeconstant. Since K may be readily found by simple experiments such astension or bending tests, the last equation reduces the determination ofP-Q at any point of a plane model 28 to the mere operation of countingfringes. The isochromatic fringe constant K for a direct transmissionpolariscope is about 300 p. s. l. per fringe for a inch model ofBakelite. For the present reflection type polariscope, the correspondingvalue of K is about 150 since the light passes twice through the model.Assuming an accuracy of a tenth fringe for ordinary inspection, thepresent plari-scope appears to be accurate to about pounds per squareinch for A inch thickness of model.

It is possible to remove the quarter-wave plates 16 and 22 from theinstrument and thereby bring into evidence the isociinic linessuperimposed upon the isochromatic lines. From the theory of isociiniclines one may find the directions of the principal stresses at eachpoint of the model.

The magneto-optic rotator operates according to the magneto-optic lawdiscovered by Michael Faraday. The light rotation may be expressed bythe equation 0=VHL where 0 is the angle through which the light vectoris rotated while traversing a length L in a transparent medium along amagnetic field H, and where V is a constant called Verdets constant.Light rotators of many difierent kinds and designs have been made thatuse this effect, but none of them have been able to give the accuratehigh degree rotation needed in an interferometer. This is largely due tothe employment in these llsht rotators of ment.

transparent cores having a low Verdets constantl The advantage of coresof this type is that they are usually stable substances. Othersubstances are known that have a higher rotational effect but thesesubstances are either unstable or semiopaque. One such substance ismercuric potassium iodide. This substance has a high rotational effectbut it is unstable and decomposes on exposure to light. By adding smallquantities of a suitable compound, it is possible to utilize thissubstance in a magnetic light rotator. By using this substance, a lightrotator of small proportions can be used. The small proportions areexceedingly important because the magnetic field should be constant andthe temperature of the rotator should be low. If the temperature of therotator rises appreciably, a thermoslphon effect will occur in thesolution and will introduce errors into the operation of the device. Itis possible to make mercuric potassium iodide a stable compound byadding from {a to /2 of 1 per cent sodium carbonate to the solution.Sodium carbonate acts as a reducing and stabilizing agent and makes thesolution stable because it prevents the oxidation that usually resultsin decomposition of the solution. The invention is not limited to theuse of sodium carbonate since it is only one of a number of reducingagents 'that act as stabilizers and could be used. The

solution of mercuric potassium iodide is held in a container 32 whichhas glass ends. The end 34 may be coated with a quarter-wave lengthcoating of barium stearate. This coating increases the lighttransmission of the glass and eliminates ghosts. The glass plate 36 atthe opposite end of the container is silvered to serve as a mirror. Thecontainer 32 has pipes 38 and 40 connected to it to permit the insertioninto the container of mercuric potassium iodide solution and also toremove all air from the solution. The container is mounted in a tube 42,which in turn is supported by end rings 44. These rings are supported byadjusting screws that permit accurate adjustment of the photoelasticinstru- It is necessary to insulate the solution from the heat of thesolenoid so the outside-of the tube 42 is covered with heat insulation.The heat insulation is enclosed by an equalizing tube. This tube extendsthe length of the insulation and is of copper or other material having ahigh thermal conductivity. The equalizing tube is encircled by a waterjacket that has pipes to connect it to water and sewage connections. The

water jacket is spaced from the equalizing tube to form an air spacethat cooperates with the equalizing tube in maintaining a uniformtemperature in the equalizing tube, A number of solenoid coils aresupported on the water jacket and are separated from each other by finsof material that has a high thermal conductivity. These fins contact thewater jacket and materially aid in dissipating the heat generated by thecoil. The magnetic intensity in the core is probably not uniform overthe cross-section of the coil. Since the light rotational effect isdependent upon the strength of the magnetic field and the length of thepath, a difference in the magnetic intensity must be accompanied by adiiierence in the length of the path to obtain uniform rotation over thewhole field of the rotator. This is done by inserting a suitable lens 33of transparent material in the condenser 32. This lens 33 has arelatively low light rotational effect and has the'same index ofrefraction as the solution. The lens 33 is curved to change the lengthof Search Roon the path through the mercuric potassium iodide. Thecurvature of the lens 33 is to insure the provision of co-axial pathsthat make the light rotation constant over the whole field of themagnetic light rotator. The curvature of the lens can be determinedaccurately by experimentation and calculation to make the product of thestrength of the magnetic intensity times the length of the path throughthe mercuric potassium iodide equal throughout the entire crosssectionalarea of the rotator. This assures constant rotation of the light. Thelens shown is believed to be usable but a concave lens may be needed.The curvature and dimensions of this lens can be determinedscientifically.

Where a test model is stressed, the surface of the model may be inclinedsuiflciently to cause the surface to change the direction of the lightas it passes through the model. This light, the direction of which hasbeen changed, will be rotated and will return to the test model but itwill not enter the model at the exact point it left. This results inerrors. The same result is had where the model is not polished finelyand these errors are quite objectionable. By submerging the test modelin a solution which has the same index of refraction, it is possible toobviate all of these errors. Furthermore, it is possible to obviate agreat deal of the polishing formerly thought necessary.

By the use of this invention, a photoelastic instrument may be made thataccurately determines the stress in test models.

Although only a preferred form of the invention has been shown anddescribed in the drawing and accompanying description, it is obvious tothose skilled in the art that various changes may be made in the form ofthe invention without affecting the scope of the invention.

What we claim is:

1. A magneto-optic light rotator comprising a solenoid coil surroundinga container having a transparent fluid therein, said container having atleast one transparent end, said container also containing a transparentlens that has at least one curved surface and has a Verdets indexdifferent from the Verdets index of said fluid, said curved surfaces ofsaid lens extending into said fluid so that it provides a plurality ofco-axial light paths for the light passing through the container thathave different eflective light rotational lengths.

2. A magneto-optic light rotator comprising a container for asubstantially transparent fluid, a

tube enclosing and supporting the said container, and rings attached tothe said tube to permit adjustment of the said rotator, insulation onthe exterior of the said tube, an equalizing tube enclosing the saidinsulation, an annular water Jacket surrounding but spaced apart fromthe said equalizing tube to form an air space therebetween, a pluralityof solenoid coils mounted on the exterior of the water jacket, saidsolenoid coils being separated from each other by cooling fins thatextend beyond the periphery of the said coils, a plurality of. pipescommunicating with the interior of the said container, andmeans tocirculate water in the water Jacket.

3. A magneto-optic light rotator for a photoelastic instrumentcomprising electrically actuated means to create a magnetic field, acontainer positioned in said field, a substantially transparent fluid insaid container, a substantially transparent article in said containerhaving a Verdets rotational constant different from that of said fluid,said article having a curved surface extending into said fluid so therotation of light resulting from the said magnetic fleld will beconstant over the area of said rotator.

4. A magneto-optic light rotator comprising a container, a transparentfluid in said container and an electrically actuated means to induce a Imagnetic field through said container, said magnetic fleid having adensity that varies with its distance from the axial center of saidcontainer, and a device having a Verdets index of rotation differentfrom said fluid that compensates for the difl'erence in the density ofsaid magnetic fleld and makes the rotation of light constant over thearea of said rotator.

FRANK W. BUBB.

ALEXANDER L. DUVAL DADRIAN.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 548,701 Crehore Oct. 29, 18951,697,451 Baird Jan. 1, 1929 1,740,673 Whitaker et al Dec. 24, 19292,144,150 Hart et al. Jan. 17, 1939 OTHER REFERENCES InternationalCritical Tables, volume 6, page 428, published in 1929.

